Then Jesus cried aloud: “Whoever believes in me believes not in me but in him who sent me. And whoever sees me sees him who sent me.” (John 12: 44-45)
John uses believe as a verb 85 times, nearly 40% of the times it is used in the New Testament (See pisteuo). It’s clearly an important concept for John (it’s roughly the same number of times it’s used in all the letters of Paul).
The verse today emphasizes that belief in Jesus equates with belief in God; that is, a right understanding of Jesus, and trusting him, is to rightly understand and trust God. This is soon after Jesus’s triumphal entry into Jerusalem, a voice from heaven affirms the message of Jesus, and Jesus stating he, the Son of Man, must be “lifted up.”
Belief, in John’s gospel, is used in a number of ways. It’s used in the sense of assent to the truth of, for example, when Martha declares she believes that Jesus is the Messiah and Son of God (John 11:27). He also uses it to mean entrust, such as when he tells his disciples not to worry, but believe (John 14:1). He often uses it to declare his equality with God, as in today’s verse. It is the starting point to an abundant life; John declares this is the reason he wrote his gospel (John 20:31). It is several times used in connection with declaring that Jesus is the light shining in darkness (eg, John 12:36).